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Abstract:

A zoom lens includes, in order from an object side to an image side, a
first lens unit having a positive refractive power, a second lens unit
having a negative refractive power, and a rear lens group including lens
units and as a whole having a positive refractive power. Distances
between the lens units change during zooming. The second lens unit
includes, in order from the object side to the image side, a negative
lens component and a cemented lens including a negative lens component
and a positive lens component. The second lens unit includes at least
five lens components. A focal length f1 of the first lens unit, a focal
length f2 of the second lens unit, a refractive index Ndp of the positive
lens component of the cemented lens, and a refractive index Ndn of the
negative lens component of the cemented lens are set appropriately.

Claims:

1. A zoom lens comprising, in order from an object side to an image side:
a first lens unit having a positive refractive power; a second lens unit
having a negative refractive power; and a rear lens group including a
plurality of lens units and as a whole having a positive refractive
power, wherein distances between adjacent ones of the lens units change
during zooming, wherein the second lens unit includes, in order from the
object side to the image side, a negative lens component; and a cemented
lens including a negative lens component and a positive lens component,
wherein the second lens unit includes at least five lens components, and
wherein the zoom lens satisfies the following conditional expressions:
5.0<|f1/f2|<9.0 1.1<Ndp/Ndn<1.5 where f1 denotes a focal
length of the first lens unit, f2 denotes a focal length of the second
lens unit, Ndp denotes a refractive index of the positive lens component
of the cemented lens, and Ndn denotes a refractive index of the negative
lens component of the cemented lens.

2. The zoom lens according to claim 1 satisfying the following
conditional expression:
0<θgFn-(0.6438-0.001682.times.νdn)<0.1 where νdn and
θgFn denote an Abbe number and a partial dispersion ratio,
respectively, of the negative lens component of the cemented lens.

3. The zoom lens according to claim 1 satisfying the following
conditional expression:
-0.1<θgFp-(0.6438-0.001682.times.νdp)<0 where νdp and
θgFp denote an Abbe number and a partial dispersion ratio,
respectively, of the positive lens component of the cemented lens.

4. The zoom lens according to claim 1, wherein the second lens unit
includes a positive lens component (lp) provided adjacent to and on the
image side of the cemented lens, and wherein the zoom lens satisfies the
following conditional expression: 1.4<Ndlp<1.7 where Ndlp denotes
a refractive index of the positive lens component (lp).

5. The zoom lens according to claim 1 satisfying the following
conditional expression: 0.5<|f2/fw|<0.8 where fw denotes a focal
length of the zoom lens as a whole at a wide-angle end.

6. The zoom lens according to claim 1, wherein the second lens unit
includes a positive lens component (lp) provided adjacent to and on the
image side of the cemented lens, and wherein the zoom lens satisfies the
following conditional expression: 2.0<|flp/f2|<4.5 where flp
denotes a focal length of the positive lens component (lp).

7. The zoom lens according to claim 1 that forms an image on a
solid-state image pickup device.

8. An image pickup apparatus comprising: the zoom lens according to claim
1; and a solid-state image pickup device that receives light representing
an image formed by the zoom lens.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a zoom lens and an image pickup
apparatus including the same, and is suitably applicable to image pickup
apparatuses such as a digital video camera, a digital still camera, and a
silver-halide-film camera.

[0003] 2. Description of the Related Art

[0004] A large-aperture zoom lens in which a lens unit having a negative
refractive power is provided nearest to an object-side end is
advantageous in performance improvement. Therefore, various proposals
concerning such a zoom lens have been provided. For example, U.S. Pat.
No. 7,184,221 discloses a large-aperture zoom lens including, in order
from an object side to an image side, a first lens unit having a negative
refractive power, a second lens unit having a positive refractive power,
a third lens unit having a negative refractive power, and a fourth lens
unit having a positive refractive power.

[0005] Japanese Patent Laid-Open No. 11-295601 discloses a large-aperture
zoom lens including, in order from an object side to an image side, a
first lens unit having a positive refractive power, a second lens unit
having a negative refractive power, a third lens unit having a positive
refractive power, and a fourth lens unit having a positive refractive
power. A zoom lens in which a first lens unit having a positive
refractive power is provided nearest to the object-side end is
advantageous in reducing the total lens length (a length from the lens
surface nearest to the object-side end to an image plane) and the lens
diameter.

[0006] A large-aperture zoom lens in which a lens unit having a negative
refractive power is provided nearest to the object-side end tends to be
large and heavy. A large-aperture zoom lens in which a lens unit having a
positive refractive power is provided nearest to the object-side end is
advantageous in size reduction, but is difficult to provide a sufficient
back focal length if the angle of view is increased.

[0007] The zoom lens disclosed by U.S. Pat. No. 7,184,221 has an f-number
as small as 2.8 and an angle of view as wide as about 84 degrees, but its
lens system is of a large size.

[0008] The zoom lens disclosed by Japanese Patent Laid-Open No. 11-295601
has an f-number as small as 2.8 with its lens system being of a small
size, but its angle of view at the wide-angle end is about 65 degrees at
most. This zoom lens is desired to have a wider angle of view.

[0009] Moreover, variations in spherical aberration and astigmatism during
zooming are not satisfactorily corrected. In addition, there remain
variations in coma aberration. With such a lens configuration and such a
refractive power arrangement, it is difficult to realize a zoom lens
having a large aperture and an angle of view that is wider than
2ω=80 degrees.

SUMMARY OF THE INVENTION

[0010] An embodiment of the present invention provides a zoom lens having
a large aperture and a wide angle of view with good optical performance
at all zooming positions, and also provides an image pickup apparatus
including the same.

[0011] A zoom lens according to an aspect of the present invention
includes, in order from an object side to an image side, a first lens
unit having a positive refractive power, a second lens unit having a
negative refractive power, and a rear lens group including a plurality of
lens units and as a whole having a positive refractive power. Distances
between adjacent ones of the lens units change during zooming. The second
lens unit includes, in order from the object side to the image side, a
negative lens component and a cemented lens including a negative lens
component and a positive lens component. The second lens unit includes at
least five lens components. The zoom lens satisfies the following
conditional expressions:

5.0<|f1/f2|<9.0

1.1<Ndp/Ndn<1.5

where f1 denotes a focal length of the first lens unit, f2 denotes a
focal length of the second lens unit, Ndp denotes a refractive index of
the positive lens component of the cemented lens, and Ndn denotes a
refractive index of the negative lens component of the cemented lens.

[0012] Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a sectional view of a zoom lens according to a first
embodiment of the present invention that is at a wide-angle end.

[0014] FIGS. 2A and 2B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the first embodiment of the present
invention at the wide-angle end and at a telephoto end, respectively.

[0015]FIG. 3 is a sectional view of a zoom lens according to a second
embodiment of the present invention that is at a wide-angle end.

[0016] FIGS. 4A and 4B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the second embodiment of the present
invention at the wide-angle end and at a telephoto end, respectively.

[0017]FIG. 5 is a sectional view of a zoom lens according to a third
embodiment of the present invention that is at a wide-angle end.

[0018] FIGS. 6A and 6B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the third embodiment of the present
invention at the wide-angle end and at a telephoto end, respectively.

[0019]FIG. 7 is a graph illustrating the relationship between Abbe number
νd and partial dispersion ratio θgF.

[0020] FIGS. 8A and 8B are diagrams illustrating the principle of
correction of lateral chromatic aberration in the zoom lens according to
any of the embodiments of the present invention.

[0021]FIG. 9 is a schematic diagram of an image pickup apparatus
according to an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0022] Embodiments of the zoom lens and the image pickup apparatus
including the same according to the present invention will now be
described.

[0023]FIG. 1 is a sectional view of a zoom lens according to a first
embodiment of the present invention that is at a wide-angle end
(short-focal-length end).

[0024] FIGS. 2A and 2B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the first embodiment of the present
invention at the wide-angle end and at a telephoto end (long-focal-length
end), respectively.

[0025]FIG. 3 is a sectional view of a zoom lens according to a second
embodiment of the present invention that is at a wide-angle end.

[0026] FIGS. 4A and 4B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the second embodiment of the present
invention at the wide-angle end and at a telephoto end, respectively.

[0027]FIG. 5 is a sectional view of a zoom lens according to a third
embodiment of the present invention that is at a wide-angle end.

[0028] FIGS. 6A and 6B are diagrams illustrating longitudinal aberration
curves in the zoom lens according to the third embodiment of the present
invention at the wide-angle end and at a telephoto end, respectively.

[0029]FIG. 7 is a graph illustrating the relationship between Abbe number
νd and partial dispersion ratio θgF.

[0030] FIGS. 8A and 8B are diagrams illustrating the principle of
correction of lateral chromatic aberration in the zoom lens according to
any of the embodiments of the present invention.

[0031]FIG. 9 is a schematic diagram of a camera (image pickup apparatus)
including the zoom lens according to any of the embodiments of the
present invention.

[0032] In the sectional views and the diagrams illustrating the
longitudinal aberration curves, the zoom lenses according to the
embodiments are each focused on an object at infinity.

[0033] The zoom lenses according to the embodiments are each an imaging
lens system that is applicable to image pickup apparatuses such as a
video camera, a digital camera, and a silver-halide-film camera.

[0034] In the sectional views of the zoom lenses, the left side
corresponds to the object side (front side), and the right side
corresponds to the image side (rear side). In the sectional views of the
zoom lenses, Li denotes an i-th lens unit, where i denotes the order of
the lens unit counted from the object side.

[0035] The zoom lenses according to the embodiments of the present
invention each include, in order from the object side to the image side,
a first lens unit having a positive refractive power, a second lens unit
having a negative refractive power, and a rear lens group including a
plurality of lens units. The rear lens group as a whole has a positive
refractive power. Distances between adjacent ones of the lens units
change during zooming.

[0036] The zoom lenses illustrated in FIGS. 1 and 5 each include a first
lens unit L1 having a positive refractive power (the reciprocal of focal
length), a second lens unit L2 having a negative refractive power, a
third lens unit L3 having a positive refractive power, and a fourth lens
unit L4 having a positive refractive power. In the zoom lenses according
to the first and third embodiments, the third lens unit L3 and the fourth
lens unit L4 in combination form a rear lens group, and the rear lens
group as a whole has a positive refractive power.

[0037] The zoom lens illustrated in FIG. 3 includes a first lens unit L1
having a positive refractive power, a second lens unit L2 having a
negative refractive power, a third lens unit L3 having a positive
refractive power, a fourth lens unit L4 having a negative refractive
power, and a fifth lens unit L5 having a positive refractive power. In
the zoom lens according to the second embodiment, the third lens unit L3,
the fourth lens unit L4, and the fifth lens unit L5 in combination form a
rear lens group, and the rear lens group as a whole has a positive
refractive power.

[0038] The above zoom lenses each include an aperture stop SP provided on
the object side of the third lens unit L3. An image plane IP is a
photosensitive surface that corresponds to an image pickup surface of a
solid-state image pickup device (photoelectric conversion device) such as
a charge-coupled-device (CCD) sensor or a
complementary-metal-oxide-semiconductor (CMOS) sensor when the zoom lens
is used as an imaging optical system of a video camera or a digital still
camera, or to a film surface when the zoom lens is used for a
silver-halide-film camera.

[0039] In the diagrams illustrating longitudinal aberration curves, d and
g denote d-line and g-line, respectively; and ΔM and ΔS
denote the meridional image plane and the sagittal image plane,
respectively. Lateral chromatic aberration is for g-line. Furthermore,
ω denotes the half angle of view, and Fno denotes the f-number.

[0040] In each of the embodiments described below, the wide-angle end and
the telephoto end refer to zooming positions at extreme ends,
respectively, of a range in which the lens units are mechanically movable
along the optical axis.

[0041] Arrows illustrated in each of the sectional views represent the
loci along which the respective lens units move during zooming from the
wide-angle end to the telephoto end. In each of the zoom lenses according
to the embodiments of the present invention, distances between adjacent
ones of the lens units change during zooming.

[0042] In each of the first and third embodiments of the present
invention, during zooming from the wide-angle end to the telephoto end,
the lens units move as represented by the respective arrows in the
following manner: the first lens unit L1 moves toward the object side;
the second lens unit L2 moves such that the distance between the second
lens unit L2 and the first lens unit L1 is increased; the third lens unit
L3 moves toward the object side such that the distance between the third
lens unit L3 and the second lens unit L2 is reduced; and the fourth lens
unit L4 moves toward the object side such that the distance between the
fourth lens unit L4 and the third lens unit L3 is reduced. The aperture
stop SP moves together with the third lens unit L3.

[0043] In the second embodiment of the present invention, during zooming
from the wide-angle end to the telephoto end, the lens units move as
represented by the respective arrows in the following manner: the first
lens unit L1 moves toward the object side; the second lens unit L2 moves
such that the distance between the second lens unit L2 and the first lens
unit L1 is increased; the third lens unit L3 moves toward the object side
such that the distance between the third lens unit L3 and the second lens
unit L2 is reduced; the fourth lens unit L4 moves toward the object side
such that the distance between the fourth lens unit L4 and the third lens
unit L3 is increased; and the fifth lens unit L5 moves toward the object
side such that the distance between the fifth lens unit L5 and the fourth
lens unit L4 is reduced. The aperture stop SP moves together with the
third lens unit L3.

[0044] Focusing is performed by moving the second lens unit L2 in an
optical-axis direction. Focusing may be performed by moving all or any
one of the lens units included in the zoom lens.

[0045] In the second embodiment of the present invention, the fourth lens
unit L4 is movable in a direction containing a component that is
perpendicular to the optical axis, whereby the image forming position is
movable in the direction perpendicular to the optical axis. In this
manner, image blurring that may occur when the zoom lens as a whole is
shaken can be corrected. That is, image stabilization is performed.

[0046] In general, to reduce the size of a lens unit, the outside diameter
(effective aperture) of the lens unit needs to be reduced. To reduce the
outside diameter of the lens unit, a light beam that is to be incident on
the lens unit needs to converge sufficiently on an incident side of the
lens unit. To make the light beam converge, another lens unit having a
strong positive refractive power can be provided on the object side of
the lens unit.

[0047] In the second embodiment of the present invention, the third lens
unit L3 and the fourth lens unit L4 move such that the distance between
the two lens units increases more at the telephoto end than at the
wide-angle end. Hence, a length sufficient for the convergence of an
axial light beam that is emitted from the third lens unit L3 is easily
provided at the telephoto end, where the diameter of the axial light beam
increases. This facilitates the size reduction of an image-stabilizing
lens unit, i.e., the fourth lens unit L4.

[0048] The optical systems, i.e., the zoom lenses, according to the
embodiments each satisfy the following conditional expression:

5.0<|f1/f2|<9.0 (1)

where f1 denotes the focal length of the first lens unit L1, and f2
denotes the focal length of the second lens unit L2.

[0049] Conditional Expression (1) appropriately defines the ratio of the
focal length of the first lens unit L1 to the focal length of the second
lens unit L2. If Conditional Expression (1) is satisfied, a retrofocus
power arrangement can be realized easily at the wide-angle end, realizing
both an increase in the angle of view at the wide-angle end and high
optical performance over the entirety of the image, with small variations
in aberrations at all zooming positions.

[0050] If the upper limit of Conditional Expression (1) is exceeded, the
refractive power of the second lens unit L2 becomes too strong, making it
difficult to reduce variations in spherical aberration and lateral
chromatic aberration that occur during zooming. Moreover, since the
effect of dispersing the axial light beam that is exerted by the second
lens unit L2 becomes too large, the size reduction of the rear lens group
becomes difficult. If the lower limit of Conditional Expression (1) is
exceeded, the retrofocus power arrangement becomes difficult to realize,
making it difficult to increase the angle of view at the wide-angle end.
Moreover, since the refractive power of the first lens unit L1 becomes
too strong, spherical aberration at the telephoto end becomes difficult
to correct.

[0051] The second lens unit L2 includes five or more lens components
including, in order from the object side to the image side, a negative
lens component and a cemented lens formed of a negative lens component
and a positive lens component.

[0052] Since the second lens unit L2 includes, in order from the object
side to the image side, a negative lens component and a cemented lens
formed of a negative lens component and a positive lens component, the
second lens unit L2 can easily have a retrofocus power arrangement,
contributing to an increase in the angle of view at the wide-angle end.
If the cemented lens includes, in order from the object side to the image
side, a biconcave lens component and a positive lens component that is
convex toward the object side, the cemented surface of the cemented lens
has a shape concentric with non-axial rays, contributing to the
correction of lateral chromatic aberration.

[0053] Since the second lens unit L2 includes five or more lens
components, aberrations that occur in the second lens unit L2, which has
a strong negative refractive power for increasing the angle of view of
the zoom lens, can be corrected in a good manner.

[0054] The zoom lenses according to the embodiments each also satisfy the
following conditional expression:

1.1<Ndp/Ndn<1.5 (2)

where Ndp denotes the refractive index of the positive lens component of
the cemented lens included in the second lens unit L2, and Ndn denotes
the refractive index of the negative lens component of the cemented lens
included in the second lens unit L2.

[0055] Conditional Expression (2) appropriately defines the ratio of the
refractive index Ndp of the positive lens component of the cemented lens
included in the second lens unit L2 to the refractive index Ndn of the
negative lens component of the cemented lens included in the second lens
unit L2.

[0056] As described above in relation with Conditional Expression (1), in
each of the embodiments of the present invention, a retrofocus power
arrangement is realized by increasing the refractive power of the second
lens unit L2, whereby the angle of view at the wide-angle end is
increased. If, however, the aperture of the zoom lens becomes large with
an f-number of about 2.8, it becomes difficult to reduce variations in
spherical aberration, in particular, chromatic spherical aberration, that
occurs during zooming.

[0057] Hence, negative spherical aberration is caused at the cemented
surface of the cemented lens such that Conditional Expression (2) is
satisfied. In this manner, positive spherical aberration that occurs in
the second lens unit L2 having a strong negative refractive power can be
reduced. Consequently, variations in spherical aberration, in particular,
chromatic spherical aberration, that occurs during zooming can be
reduced.

[0058] If the upper limit of Conditional Expression (2) is exceeded and
the ratio of the refractive index of the positive lens component to the
refractive index of the negative lens component becomes too large, the
Petzval sum of the second lens unit L2 becomes a large negative value,
making it difficult to reduce variations in the field curvature that
occurs during zooming. If the lower limit of Conditional Expression (2)
is exceeded and the ratio of the refractive index of the positive lens
component to the refractive index of the negative lens component becomes
small, the effect of correcting the spherical aberration at the cemented
surface is reduced, making it difficult to reduce variations in spherical
aberration, in particular, chromatic spherical aberration, that occurs
during zooming.

[0059] The ranges of Conditional Expressions (1) and (2) may be set as
follows:

6.0<|f1/f2|<8.5 (1a)

1.1<Ndp/Ndn<1.3 (2a)

[0060] If Conditional Expressions (1a) and (2a) are satisfied, it becomes
easy to reduce aberrations at all zooming positions while further
increasing the angle of view.

[0061] The zoom lenses according to the embodiments each can produce
advantageous effects that correspond to respectively different
conditional expressions given below. Each zoom lens may satisfy one or
more of the conditional expressions.

[0063] Then, the zoom lens may satisfy the following conditional
expression:

0<θgFn-(0.6438-0.001682×νdn)<0.1 (3)

where νdn and θgFn denote the Abbe number and the partial
dispersion ratio, respectively, of the material of the negative lens
component of the cemented lens included in the second lens unit L2.

[0064]FIG. 7 is a graph illustrating the relationship between the Abbe
number νd and the partial dispersion ratio θgF of an optical
glass material. In FIG. 7, point A represents a case of a product named
PBM2 (νd=36.26 and θgF=0.5828) manufactured by Ohara Inc., and
point B represents a case of a product named NSL7 (νd=60.49 and
θgF=0.5436) manufactured by Ohara Inc. With respect to a reference
line connecting point A and point B, high-dispersion glass materials each
having an Abbe number νd smaller than 35 tend to be plotted above the
reference line, whereas low-dispersion glass materials each having an
Abbe number νd of 35 to about 60 tend to be plotted below the
reference line. Some anomalous-dispersion glass materials each having an
Abbe number νd of 60 or greater are plotted above the reference line.
Among low-dispersion glass materials, materials that are plotted above
the reference line are effective in the correction of secondary spectrum,
and the effect of the correction increases as the point on the graph goes
away from the reference line.

[0065] To correct lateral chromatic aberration in a good manner at all
zooming positions, the coefficient of lateral chromatic aberration of the
zoom lens as a whole needs to be controlled so as to be a value close to
zero at all zooming positions. Here, a coefficient of lateral chromatic
aberration T is expressed as follows:

T=Σ(h˜hbφ/νd)

where φ denotes the refractive power of the zoom lens, h denotes the
height of incidence of an axial ray, hb denotes the height of incidence
of a non-axial principal ray, and νd denotes the Abbe number. Hence,
lateral chromatic aberration is dominantly influenced by a lens unit in
which the height of incidence of a non-axial principal ray hb varies
greatly. The influence of the second lens unit L2, in which the absolute
value of the refractive power φ is large, is the second largest.

[0066] FIGS. 8A and 8B are diagrams illustrating the principle of
correction of lateral chromatic aberration in the zoom lens according to
any of the embodiments of the present invention. The zoom lens
illustrated in FIGS. 8A and 8B includes, in order from the object side to
the image side, a first lens unit L1 having a positive refractive power,
a second lens unit L2 having a negative refractive power, and a rear lens
group having a positive refractive power. As the focal length of the zoom
lens as a whole becomes larger, the distance between the first lens unit
L1 and the second lens unit L2 increases while the distance between the
second lens unit L2 and the rear lens group is reduced. A non-axial
principal ray that is incident on this zoom lens will now be studied. At
the wide-angle end, the non-axial principal ray travels as illustrated in
FIG. 8A. At the telephoto end, the non-axial principal ray travels as
illustrated in FIG. 8B. The lens units provided on the image side of the
aperture stop SP are not illustrated in FIGS. 8A and 8B.

[0067] In the known zoom lenses, if lateral chromatic aberrations for
g-line and C-line are corrected in such a manner as to occur at the same
position on the image plane, that position is shifted, with respect to a
position of occurrence of lateral chromatic aberration for d-line, in a
direction away from the optical axis at the wide-angle end and in a
direction toward the optical axis at the telephoto end.

[0068] The above lateral chromatic aberration for g-line is corrected on
the basis of the following principle. In a case where the negative lens
component of the second lens unit L2 having a negative refractive power
is made of an anomalous-dispersion glass material, a force that bends
g-line toward the optical axis becomes stronger. This is because the
refractive power of the anomalous-dispersion glass material for g-line is
relatively higher than that of a normal lens material. Here, the height
of incidence of a non-axial principal ray hb is smaller at the telephoto
end than at the wide-angle end. Accordingly, the influence of the second
lens unit L2 becomes smaller than that at the wide-angle end. That is, if
Conditional Expression (3) is satisfied, the increase in lateral
chromatic aberration at the telephoto end is suppressed to some extent.
Hence, the secondary spectrum of lateral chromatic aberration at the
telephoto end can be improved significantly.

[0069] If the lower limit of Conditional Expression (3) is exceeded, the
anomalous dispersion caused by the material of the negative lens
component of the cemented lens included in the second lens unit L2
becomes small, making it difficult to satisfactorily reduce lateral
chromatic aberration at the wide-angle end. If the upper limit of
Conditional Expression (3) is exceeded, the anomalous dispersion caused
by the material of the negative lens component of the cemented lens
becomes too large, making it difficult to correct longitudinal chromatic
aberration.

[0070] The zoom lenses according to the embodiments may each satisfy the
following conditional expression:

-0.1<θgFp-(0.6438-0.001682×νdp)<0 (4)

where νdp and θgFp denote the Abbe number and the partial
dispersion ratio, respectively, of the positive lens component of the
cemented lens included in the second lens unit L2.

[0071] If Conditional Expression (4) is satisfied, the increase in lateral
chromatic aberration at the telephoto end is suppressed to some extent.
Hence, the secondary spectrum of lateral chromatic aberration at the
telephoto end can be improved significantly.

[0072] If the lower limit of Conditional Expression (4) is exceeded, the
anomalous dispersion caused by the material of the positive lens
component of the cemented lens becomes too large, making it difficult to
correct longitudinal chromatic aberration. If the upper limit of
Conditional Expression (4) is exceeded, the anomalous dispersion caused
by the material of the positive lens component of the cemented lens
becomes small, making it difficult to satisfactorily reduce lateral
chromatic aberration at the wide-angle end.

[0073] In a case where the second lens unit L2 includes a positive lens
component lp provided adjacent to and on the image side of the cemented
lens, the zoom lens may satisfy the following conditional expression:

1.4<Ndlp<1.7 (5)

where Ndlp denotes the refractive index of the positive lens component
lp.

[0074] If Conditional Expression (2) is satisfied, variations in spherical
aberration, in particular, chromatic spherical aberration, that occurs
during zooming can be reduced. However, the Petzval sum of the second
lens unit L2 tends to become a large negative value. In such a case, if
Conditional Expression (5) is satisfied, the Petzval sum of the second
lens unit L2 can be corrected in a good manner.

[0075] If the upper limit of Conditional Expression (5) is exceeded, the
Petzval sum of the second lens unit L2 becomes an excessively large
negative value, making it difficult to reduce variations in the field
curvature that occurs during zooming. If the lower limit of Conditional
Expression (5) is exceeded, it becomes difficult to correct spherical
aberration at the telephoto end.

[0076] The zoom lens may satisfy the following conditional expression:

0.5<|f2/fw|<0.8 (6)

where f2 denotes the focal length of the second lens unit L2, and fw
denotes the focal length of the zoom lens as a whole at the wide-angle
end.

[0077] Conditional Expression (6) defines the focal length of the second
lens unit L2. If the upper limit of Conditional Expression (6) is
exceeded, the length of travel of the first lens unit L1 during zooming
needs to be increased. Consequently, the total lens length at the
telephoto end increases disadvantageously, or, since the length of travel
of the first lens unit L1 during zooming is increased, it becomes
difficult to reduce the size of the zoom lens as a whole.

[0078] If the lower limit of Conditional Expression (6) is exceeded, the
zoom ratio is advantageously increased. However, the Petzval sum becomes
negatively large, making it difficult to correct astigmatism at all
zooming positions.

[0079] In a case where the second lens unit L2 includes a positive lens
component 1p provided adjacent to and on the image side of the cemented
lens, the zoom lens may satisfy the following conditional expression:

2.0<|flp/f2|<4.5 (7)

where flp denotes the focal length of the positive lens component lp.

[0080] Conditional Expression (7) defines the focal length of the positive
lens component lp included in the second lens unit L2. If the upper limit
of Conditional Expression (7) is exceeded, the refractive power of the
positive lens component lp becomes too weak and the Petzval sum of the
second lens unit L2 becomes an excessively large negative value, making
it difficult to reduce variations in field curvature that occurs during
zooming. If the lower limit of conditional Expression (7) is exceeded, it
becomes difficult to correct spherical aberration at the telephoto end.

[0081] In each of the embodiments, to further reduce variations in
aberrations that occur during zooming and the size of the zoom lens while
correcting aberrations in a good manner, the ranges of Conditional
Expressions (3) to (7) may be set as follows:

0<θgFn-(0.6438-0.001682×νdn)<0.02 (3a)

-0.02<θgFp-(0.6438-0.001682×νdp)<0 (4a)

1.45<Ndlp<1.65 (5a)

0.60<|f2/fw|<0.75 (6a)

2.0<|flp/f2|<4.0 (7a)

[0082] According to each of the above embodiments, a zoom lens having a
large aperture and a wide angle of view with good optical performance at
all zooming positions is provided.

[0083] Numerical Examples 1 to 3 corresponding to the respective first to
third embodiments are given below. In each of Numerical Examples 1 to 3,
i denotes the order of the surface counted from the object side; ri
denotes the radius of curvature of the i-th surface; di denotes the
distance between the i-th surface and the i+1-th surface; ndi and νdi
denote the refractive index and the Abbe number, respectively, with
respect to d-line; f denotes the focal length; and Fno denotes the
f-number.

[0084] Data on aspherical surfaces are the coefficients of aspherical
surfaces when the aspherical surfaces are each expressed as follows:

where x denotes the displacement in the optical-axis direction from a
reference surface, h denotes the height in a direction perpendicular to
the optical axis, R denotes the radius of a quadric surface as the base,
k denotes the conic constant, and Cn denotes the n-th-order coefficient
of the aspherical surface.

[0085] In addition, an expression "E-Z" denotes "10-z".

[0086] Furthermore, Table 1 summarizes the relationships between
Conditional Expressions (1) to (7) given above and values in Numerical
Examples 1 to 3.

[0090] An embodiment in which the zoom lens according to any of the above
embodiments of the present invention is used as an imaging optical system
will now be described with reference to FIG. 9. FIG. 9 illustrates a body
10 of a single-lens reflex camera with an interchangeable lens 11 to
which the zoom lens according to any of the embodiments of the present
invention is applied.

[0091] A photosensitive surface 12 corresponds to a silver-halide film, a
solid-state image pickup device (photoelectric conversion device), or the
like. An image of an object obtained through the interchangeable lens 11
is to be recorded on the silver-halide film or is to be received by the
solid-state image pickup device.

[0092] The image of the object obtained through the interchangeable lens
11 is viewed through a finder optical system 13. The image of the object
obtained through the interchangeable lens 11 is switchably transmitted to
either the photosensitive surface 12 and the finder optical system 13 via
a quick return mirror 14 that is turnable.

[0093] When an image of an object is to be viewed through the finder
optical system 13, the image of the object formed on a focusing screen 15
via the quick return mirror 14 is converted into an erect image by a
pentagonal prism 16 and is magnified by an eyepiece optical system 17,
whereby the magnified image is viewed.

[0094] When image shooting is performed, the quick return mirror 14 turns
in the direction of the arrow. Thus, the image of the object is formed
and recorded on the photosensitive surface 12.

[0095] By applying the zoom lens according to any of the embodiments of
the present invention to an optical apparatus such as an interchangeable
lens of a single-lens reflex camera as described above, an optical
apparatus having high optical performance is provided.

[0096] The present invention is also applicable in a similar manner to a
single-lens reflex camera not including a quick return mirror.

[0097] The zoom lenses according to the embodiments of the present
invention is also applicable in a similar manner to a video camera.

[0098] While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0099] This application claims the benefit of Japanese Patent Application
No. 2012-023338 filed Feb. 6, 2012, which is hereby incorporated by
reference herein in its entirety.